Written By Dr. Kris Hiney

Last month we discussed the use of probiotics in horses, including the definition and types of probiotics, their effectiveness, and when their use might be warranted.  This month we will address a closely related and often misunderstood topic: prebiotics.  The use of both of these feed additives may work synergistically to promote digestion in your horse, keep his immune system in top shape and allow him to face the various stressors which may be present in his life.

As opposed to probiotics, prebiotics are not live organisms.  Rather, they are chains of specific types of carbohydrates which promote the growth of organisms which are beneficial to the well-being of the host.  Prebiotics are derived from a variety of products, including milk, fruit, vegetables and fermentation byproducts.  These are typically short chains of fructo-oligosaccharides (FOS), which are a mix of fructose and glucose, mannose oligo-saccharides (MOS) or galacto-oligosaccharides (GOS).  Simply stated, oligosaccharides are shorter chains of carbohydrates or saccharides (sugars) as compared to polysaccharides such as starch, glycogen of cellulose.  For a review of carbohydrate terminology, please see: Equine Carbohydrate Disorders Part 1. Because of the type of bonds joining the carbohydrates together, prebiotics are not able to be enzymatically digested in the stomach and small intestine.  Instead they provide substrates for fermentation of a specific group of bacteria and thus allow them to flourish.  It may be helpful to think about prebiotics as providing food for the good types of bacteria, rather than feeding your horse.   In ruminant animals such as cattle, sheep and goats, they have a direct effect on the rumen microflora, while in monogastrics and hind gut fermenters such as the horse, prebiotics pass to the hindgut where they exert part of their beneficial effect.   Horses also have a substantial microbial population in the foregut as well.  While the use of prebiotics in gastric health of the horse has not been explored, it does appear promising as a potential tool in maintaining stomach health.

So why would you have to feed the bacteria?  Certainly a horse on a high forage diet would have adequate nutrient delivery to those microbes, correct?  Well, different types of organisms utilize different substrates for food.  If there is more food available for one type, they will be more successful and reproduce at a higher rate.  Cellulytic bacteria are responsible for fermentation of the fibrous portion of a horse’s feed and are typically associated with a higher pH in the gut.   However, amylotic bacteria prefer substrates such as sugars and starches. When we over feed concentrate to our horse or forages containing more rapidly fermentable sugars, these amylotic bacteria flourish and can produce negative by products for the horse.  Prebiotics typically provide substrate for more beneficial strains of bacteria including bifidobacteria (found more in humans), lactobacillus and lactate utilizing bacteria.  The use of prebiotics has shown to be effective in preventing the rapid and detrimental shift in bacterial population which occurs when horses are overfed concentrates. Now certainly it would never be advisable to subject our horses to a rapid increase in carbohydrates.  However, we could think of supplementation of prebiotics during periods of dietary adaptation, shifting to a new feed source or when starting to graze in the spring as a potential way to modulate gut microflora. However, supplementation of prebiotics would not be an alternative to slow controlled adaptation to new diets.

Prebiotics may have more benefits than just helping to increase fermentation or stabilize the population of the hindgut.  While not digested in the small intestine, prebiotics help prevent the colonization of pathogenic bacteria such as Salmonella and E coli.  Doing so improves the overall health status of the animal.  They do so by binding to the carbohydrate receptor sites on the bacteria which are used to bind to intestinal epithelial cells. By “tricking” bacteria into binding to these oligosaccharides, they are prevented from binding to epithelial cells and proliferating. Promoting the growth of the beneficial bacteria may even limit the growth of pathogenic bacteria.  Bifidocacteria and lactobacillus possess their own bactericidal/anti-microbial effects against harmful bacteria. The “good” type of bacteria may also release enzymes which destroy the toxins produced by pathogenic bacteria.  Clearly it is easy to see why the feeding of prebiotics has gained much attention in feeding production species as an alternative to antibiotics.

In addition to these direct effects on bacteria, immune-stimulatory effects of prebiotics have been observed in a variety of subjects. These appear to be not only in response to viral or bacterial challenge, but even with allergen stimulated immune responses. Human infants supplemented with prebiotics which mimic those found in breast milk developed fewer infections compared to those not receiving prebiotics.  There is also some evidence that intestinal epithelial cells may be able to transport prebiotic oligosaccharides, putting them in direct contact with cells of the immune system.  In an in vitro equine study, peripheral blood mononuclear cells (lymphocytes, macrophages) showed an increased immune response when these cells were cultured in the presence of GOS and FOS. When these same cells were challenged with LPS, the effect was even more pronounced in cells cultured with FOS and GOS.  The use of prebiotics may be a tool in helping to develop the immune system of neonatal foals, as has been proposed in other species.    Finally, prebiotics may serve as natural anti-oxidants themselves. In part this could help explain their immunomodulatory effects as well.  Therefore, consider using prebiotics when the animal might be undergoing periods of stress, as stress typically weakens the immune system.

Even beyond their ability to affect the population of micro-organisms in the hindgut or stimulate the immune system, prebiotics may also help with insulin sensitivity.  This has been shown in dogs, veal calves, rodents and humans.   The effect is believed to be due to the alteration of fermentation in the hindgut, resulting in a shift in the ratio of volatile fatty acids which are produced. In obese horses supplemented with short chain FOS, a modest improvement in insulin sensitivity was observed after 6 weeks.  Prebiotics may serve as an aid to restoring insulin sensitivity, but certainly should not replace diet modifications or a sound weight loss program.

Certainly the addition of prebiotics to the human food supply is increasing, and a number of products designed for use in pet foods and production animals point to the value of this natural foodstuff in promoting the health for all. There do not appear to be any risks associated with feeding prebiotics, and the number of proven health benefits is quite expansive.  The evidence for their effectiveness in improving the health and well-being is so many species of animals is substantial.  So if your horse needs help with digestion, stress, his immune system or even insulin resistance, consider a prebiotic.